X-ray observations of the hot phase in Sagittarius A*

Context. We analyse 134 ks Chandra ACIS-I observations of the Galactic Centre (GC) performed in July 2011. The X-ray image with the field of view 17′ × 17′ contains the hot plasma surrounding the Sgr A*. The obtained surface brightness map allows us to fit the Bondi hot accretion flow to the innermost hot plasma around the GC. Aims. Contrary to the XMM-Newton data where a strong 6.4 keV iron line was observed andinterpreted as a reflection from molecular clouds, we search here for the diffuse X-ray emission with prominent 6.69 keV iron line. We found a surface brightness profile that allowed us to determine the stagnation radius of the flow around Sgr A*. Methods. We fitted spectra from the region up to 5′′ from Sgr A* using a thermal bremsstrahlung model and four Gaussian profiles responsible for Kα emission lines of Fe, S, Ar, and Ca. The X-ray surface brightness profile up to 3′′ from Sgr A* found in our data image was successfully fitted with the dynamical model of Bondi spherical accretion. Results. We show that the temperature of the hot plasma derived from our spectral fitting is of the order of 2.2−2.7 keV depending on the choice of background. By modelling the surface brightness profile, we derived the temperature and number density profiles in the vicinity of the black hole. The best fitted model of spherical Bondi accretion shows that this type of flow works only up to 3′′ and implies that the outer plasma density and temperature are n e = 18.3 ± 0.1 cm−3 and T out e = 3.5 ± 0.3 keV, respectively. Conclusions. We show that the Bondi flow can reproduce observed surface brightness profiles up to 3′′ from Sgr A* in the Galactic centre. This result strongly suggests the position of stagnation radius in the complicated dynamics around GC. The temperature at the outer radius of the flow is higher by 1 keV than the value found by our spectral fitting of thermal plasma within the circle of 5′′. The Faraday rotation computed from our model towards the pulsar PSR J1745-2900 near the GC agrees with the observed one, recently reported. We speculate here that the emission lines observed in spectra up to 5′′ can be interpreted as the reflection of the radiation from two-phase regions occurring at distances between 3−5′′ of the flow. The hot plasma in Sgr A* illuminated in the past by a strong radiation field may be a seed for thermal instabilities and eventual strong clumpiness.